Embodiments of the present invention relate to gas turbine engine fuel nozzles and, more particularly, to apparatus for draining and purging gas turbine engine fuel nozzles.
Aircraft gas turbine engines include a combustor in which fuel is burned to input heat to the engine cycle. Typical combustors incorporate one or more fuel injectors whose function is to introduce atomized, liquid fuel into an air flow stream at the combustor inlet so that it can be burned effectively to produce necessary heat for the cycle.
Staged combustion systems have been developed to limit the production of undesirable combustion product components such as oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO). Other factors that influence combustor design are the desires of users of gas turbine engines for efficient, low cost operation, which translates into a need for reduced fuel consumption while at the same time maintaining or even increasing engine output. As a consequence, important design criteria for aircraft gas turbine engine combustion systems include provisions for high combustion temperatures, in order to provide high thermal efficiency under a variety of engine operating conditions, as well as minimizing undesirable combustion conditions that contribute to the emission of particulates, and to the emission of undesirable gases, and to the emission of combustion products that are precursors to the formation of photochemical smog.
In a staged combustion system, the nozzles of the combustor are operable to selectively inject fuel through two or more discrete stages, each stage being defined by individual fuel flowpaths within the fuel nozzle. For example, the fuel nozzle may include a pilot stage that operates continuously, and a main stage that only operates at higher engine power levels. The fuel flowrate may also be variable within each of the stages.
A significant concern in this type of fuel nozzle is the formation of carbon (or “coke”) deposits when a liquid hydrocarbon fuel is exposed to high temperatures in the presence of oxygen. This process is referred to as “coking” and is generally a risk when temperatures exceed about 177° C. (350° F.). When normal staged operations stops flow to one of the aforementioned stages, a volume of fuel will continue to reside in the fuel nozzle and can be heated to coking temperatures. The areas of highest concern relative to coking are small main injection orifices within the fuel nozzle, where the fuel increases temperature most rapidly when main fuel flow is off due to staging. Small amounts of coke interfering with fuel flow through these orifices can make a large difference in fuel nozzle performance. Eventually, build-up of carbon deposits can block fuel passages sufficiently to degrade fuel nozzle performance or prevent the intended operation of the fuel nozzle to the point where cleaning or replacement is necessary to prevent adverse impacts to other engine hot section components and/or restore engine cycle performance.
Prior art designs have addressed this problem by purging the complete main fuel circuit of liquid fuel when the main stage was not operating. While effective, this type of complete purge requires motive pressure differentials with magnitude proportional to the length of the passage to be purged and could cause relatively long delays in refilling the main fuel circuit when high power operation was again desired.
Accordingly, it would be desirable to have a method of purging a portion of a fuel nozzle stage when that stage is not in operation.